1. Field of the Invention
The present invention generally relates to the field of optics, and more particularly to the protection of eyes, sensors, and other objects from high intensity light.
2. Description of the Related Art
High intensity light sources, such as lasers, electric arc welders, and direct sunlight, are potentially damaging to the human eye as well as to optoelectronic sensors and other equipment used in technological environments. Workers in such environments have conventionally worn eye protection glasses or goggles including lenses formed of a material having a high level of optical absorption for all values of incident light intensity. The lenses in welding goggles and face plates, for example, absorb so much light under normal conditions that practically nothing can be seen through them until the welding arc is struck, requiring them to be constantly moved into and away from their protecting position in front of the worker's eyes.
Optical limiters whose level of absorption increases as the incident light intensity increases have been developed using materials which exhibit reverse saturable absorption (RSA). These materials have conventionally been dissolved in a liquid or incorporated into a solid with a uniform concentration of molecules. The principles of RSA are outlined in a paper entitled "An analysis of pulse propagation through a saturable absorber having excited-state absorption", by K. Reddy, in Optical and Quantum Electronics 19 (1987), pp. 203-208.
Optical limiting action can be enhanced by converging an input light beam into the material, as described in a paper entitled "PASSIVE BROADBAND HIGH DYNAMIC RANGE SEMICONDUCTOR LIMITERS", by D. Hagan et al, in SPIE vol. 1105 Materials for Optical Switches, Isolators, and Limiters (1989), pp. 103-113. The material reported was a semiconductor, polycrystalline ZnSe.
Optical limiters using reverse saturable absorbers in the past have all had a uniform concentration of molecules throughout the extent of the absorbing material. The disadvantage of this, whether or not the input light beam is converged into the material, is that many of the molecules are outside the focal region (high fluence region), and will act as normal linear absorbers and not contribute to the optical limiting action. This is because the local fluence, or energy flux per unit area, in the material outside the focal region, is below the threshold at which the reverse saturable absorption "turns on", and involvement of the triplet levels begins to occur as discussed above.